Switchgear, switchboards, panel boards and other assemblies are general terms which cover metal enclosures or cabinets that house switching and interrupting devices such as fuses and circuit breakers along with associated control, instrumentation and metering devices. Such assemblies typically include buses, interconnections and supporting structures used for distribution of electric power. In addition, the assemblies are categorized as high, medium and low voltage switchgear and switchboards. Low voltage switchgear and switchboards operate at voltages up to 1000 volts and with continuous currents up to 6000 amperes. They are designed to withstand short-circuit currents up to 200,000 amperes.
Typical switchgear equipment includes a lineup of multiple metal enclosed sections. Each section may have several circuit breakers stacked one above the other vertically in the front of the section with each breaker being enclosed in its own metal compartment. Each section has a vertical or section bus which supplies current to the breakers within the section via short horizontal branch buses, also referred to as run-in buses, which extend through insulated openings in the rear wall of the breaker compartments. The vertical buses in each section are supplied with current by a horizontal main bus that runs through the line-up. The rear of the section is typically an open area for the routing of cables.
It has recently become desirable to provide low voltage switchgear with circuit breakers having 6000 ampere continuous current ratings. In order to comply with American National Standards Institute Standard C37.50 (ANSI standard), the temperature of the switchgear must be within a maximum temperature limit at 100% rated current. In accordance with the ANSI standard, the temperature is measured throughout the switchgear including the power circuit breakers. However, additional heat is generated in the switchgear due to the increase of circuit breaker current to 6000 amperes. In particular, since heat generation is directly proportional to the square of the current, the increase in current results in an increase of approximately 44% in the amount of heat that is generated at 100% rated current. However, the same temperature limits set forth in the ANSI standard have to be met without increasing the physical size of the power circuit breaker or the footprint of the switchgear. Currently available switchgear equipment is air cooled through natural convection by utilizing vents in the back and top of switchgear. However, it has been found that cooling by natural convention is not sufficient to accommodate the additional heat generated by the increased current.